This invention relates to a method for extracting pin bones from a fillet. More particularly it concerns a method for extracting pin bones from a fillet where the pin bones are extracted by a tool having a certain displacement speed relative to the fillet. The invention also comprises a device for performing the method.
From the dorsal vertebra of a fish extends, as is known, a vertically upward extending bone, and concerning the anterior dorsal vertebrae also at least two ribs encircling the abdominal cavity. In several kinds of fish, among them salmonids, are the anterior dorsal vertebrae also provided with two so-called dorsal ribs extending mainly horizontally out in different directions from the dorsal vertebra. The dorsal is ribs are often referred to as pin bones. The said directions are referred to the normal swimming position of the fish.
The pin bone comprises a relatively hard bone portion closest to the dorsal vertebra and a softer gristly portion comprising collagen fibres closer to its outer end portion. The outer end portion is provided with a relatively large number of threadlike soft tendons extending from the gristle portion and out into the fish flesh. These tendons connect the pin bone to the fish flesh in an efficient way.
When the pin bone during gutting of fresh fish is to be extracted from the fish flesh, the said relatively strong connection between the pin bone and the fish flesh causes that the pin bone may be torn in a position along its complete length, or that some of the fish flesh may come with the pin bone out of the fish. If fish flesh comes out with the pin bone, this means loss of fish flesh yield, an opening in the fillet and damage to the fish flesh.
To overcome this problem it is common according to prior art to slice the fish flesh possibly V-shaped along the pin bones to be able to remove them, or to wait until after the death stiffness has gone out of the flesh and then extract them.
Experience shows that the speed at which the pin bone is extracted from the fish flesh is of great importance to achieve is a successful removal of the pin bones. At too speedy extraction the pin bones are torn or pull with them fish flesh while they at a relatively slow pulling speed let go the fish flesh and may be removed in an expedient manner.
Localization of pin bones and the extraction operation itself may be carried out by methods and devices known to a person well versed in the art and are therefore not described further.
Due to the relatively slow speed at which the pin bones may be extracted it is often necessary to spread fish fillets from a production line between two or more pin bone extractors to be able to utilise the capacity of the production line. Such spreading complicates matters and leads thereby to additional costs.